1. Changed ModuleBuild's context property to db column, it's
non-nullable and default value is '00000000' to keep it consistent
with previous behaviour.
2. Changed ModuleBuild.contexts_from_mmd to return a tuple of
(ref_build_context, build_context, runtime_context, context).
3. Updated tests affected by this change.
- DB migration scripts.
- Models: ComponentBuildTrace, ModuleBuildTrace.
- ModuleBuild.state_trace method for querying for a particular module's
state history.
- SQLAlchemy before commit session event handler for recording
module/component state changes.
- REST API verbose mode for getting state trace of a particular module.
- Tests use make_session, so that event handlers are in effect.
- Short info in README about verbose mode.
- Tests verifying whether state trace information about a module appears
in verbose mode.
- Other minor fixes (RidaBase -> MBSBase, PEP8...)
